Fluorocarbons, particularly fluorinated olefins, as a class, have many and varied uses, including as chemical intermediates and monomers. In particular, these products are useful as refrigerants, monomers or intermediates for preparing refrigerants, particularly those identified as having low global warming potential.
With concerns over global warming, hydrofluoroolefins (HFOs) are being commercialized as substitutes for chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) for use as refrigerants, heat transfer agents, blowing agents, monomers and propellants because HFOs do not deplete the ozone layer and have low global warming potential. Some HFOs are prepared by multiple steps that involve fluorinating a chlorinated organic compound with a fluorination agent such as hydrogen fluoride in the presence of a fluorination catalyst. These reactions may be conducted in either the liquid or gas phase or a combination of these. Among processes to manufacture 2,3,3,3-tetrafluoropropene (HFO-1234yf), the following reaction sequence is known:TCP+3HF→1233xf+3HCl  Step 1wherein TCP is 1,1,2,3-tetrachloropropene, or CCl2═CClCH2Cl and/or 2,3,3,3,-tetrachloropropene or CH2═CClCCl3; and 1233xf is 2-chloro-3,3,3,-trifluoropropene, or CH2═CClCF3;1233xf+HF→244bb  Step 2wherein 244bb is 2-chloro-1,1,1,2-tetrafluoropropane, or CH3CClFCF3.
A by-product of Step 2 can also form as follows: 1233xf+2HF→245cb+HCl, where 245cb is 1,1,1,2,2-pentafluoropropane, or CH3CF2CF3; and244bb→1234yf+HCl  Step 3wherein 1234yf is 2,3,3,3-tetrafluoropropene, or CH2═CFCF3.
In various practices, Step 1 takes place in the gas phase in the presence of a fluorination catalyst, Step 2 takes place in the liquid phase in the presence of a fluorination catalyst, and Step 3 takes place in the gas phase in the presence or absence of a dehydrochlorination catalyst.
For Step 2 of the above process, herein referred to as “the Step (2) reaction,” liquid phase fluorination is preferred because the reaction can be controlled at relatively lower temperatures, which, in turn, results in less by-product formation due, e.g., to decomposition. In the liquid phase fluorination of HCFO-1233xf to produce HCFC-244bb, no HCl, or at least no meaningful amount, is produced because the reaction is strictly a hydrofluorination reaction where HF adds across the double bond of 1233xf. This lack of meaningful HCl by-product formation is unique when compared to other well-known liquid phase fluorination reactions that produce CFCs (e.g. CFC-12), HCFCs (e.g. HCFC-22, HCFC-142b), and HFCs (e.g. HFC-143a, HFC-245fa). This is because these reactions involve a halogen exchange, in whole or in part. That is, F− replaces a Cl− on the molecule.
However, it has been found that the inadequate HCl formation has consequences that are adverse to processing. For example, it has been found that because inadequate HCl is produced in the reaction of HCFO-1233xf to HCFC-244bb, there is less mixing in the reactor, which can decrease conversion and also promote by-product formation. In addition, the reactor itself is more difficult to control; among other reasons for this is difficulty in achieving and/or maintaining adequately elevated pressures in the reactor needed to help carry out the reaction to form HCFC-244bb. It has been found in this regard that the formation of HCl in sufficient quantities correlates with the generation of higher pressure, and that the lack of sufficient HCl causes deficient pressure. Other advantages ascribable to the adequate formation of HCl have been found as well. For example, because it is non-condensable at the desired reaction conditions, adequate HCl formation would also increase mixing in the reactor; also, it would readily be carried out in the overhead of the catalyst stripper, and would help carry out the fluorinated product.
Thus there is a need to compensate for the inadequate formation of HCl formation in Step (2) of the reaction.